This invention relates to an exposure apparatus to be used with an exposure beam of ultraviolet rays, such as an excimer laser, for example, and also to a device manufacturing method using such an exposure apparatus.
In projection exposure apparatuses for the manufacture of semiconductor integrated circuits, light of various wavelength regions is projected as an exposure beam to a substrate. Examples of such an exposure beam used are e-line (wavelength xcex=546 nm), g-line (xcex=436 nm), h-line (xcex=405 nm), i-line (xcex=365 nm), KrF excimer laser (xcex=248 nm), ArF excimer laser (xcex=193 nm) and X-rays.
An exposure beam emitted from a light source goes through an illumination optical system for illuminating a mask or reticle, and a projection optical system (projection lens) for imaging a fine pattern, formed on the mask, whereby the fine pattern is lithographically transferred to a photosensitive substrate. In such exposure apparatuses, the linewidth of a pattern becomes smaller and smaller, and this has forced further improvement of throughput and resolution. Also, this requires a large power exposure beam. Simultaneously, the bandwidth of a wavelength of the exposure beam should be narrowed.
It is known that, when an exposure beam of i-line or a wavelength shorter than it is used, due to the band-narrowing, impurities in the air photochemically react with oxygen to cause deposition of compositions (blurring material) produced by the reaction upon optical elements (lenses or mirrors) of the optical system. This produces non-transparent xe2x80x9cblurxe2x80x9d.
A representative example of such a blurring material is ammonium sulfate (NH4)2SO4, resulting from that, when sulfurous acid, for example, absorbs energies of light and is excited thereby, it reacts with oxygen in the air (i.e., oxidized). The ammonium sulfate is colored white so that, when it is deposited on an optical member such as a lens or mirror, it causes xe2x80x9cblurxe2x80x9d. Then, the exposure beam is scattered or absorbed by ammonium sulfate and, as a result, the transmission factor of the optical system decreases. This causes a reduction of light quantity (transmission factor) reaching the photosensitive substrate, and a decrease of throughput.
As a solution for this problem, Japanese Laid-Open Patent Application, Laid-Open No. 216000/1994 shows an apparatus wherein a barrel comprises a casing of a closed structure having glass members such as lenses accommodated therein, and wherein the inside of the casing is filled with an inert gas.
As regards an illumination optical system for illuminating a mask with a laser light source and a projection optical system for projecting the pattern of the mask in a reduced scale, enclosing optical components such as lenses by a tightly closed container and purging the inside of the container with an inert gas, may be accomplished relatively easily. However, as regards the mask and the substrate, particularly, the substrate has to be changed frequently. From the standpoint of throughput, therefore, it is practically difficult to place the substrate in a space purged by an inert gas. The space around the substrate inevitably contains air. When oxygen (O2) in the air absorbs the exposure beam, ozone (O3) is produced.
Generally, a temperature control system for the major assembly of the apparatus is structured to circulate a portion of the air conditioning gas, not to discharge all the gas outside the apparatus. Therefore, ozone successively produced in response to exposures remain in the temperature control system, and the ozone density in the apparatus gradually increases to some extent. As a result of it, the surfaces of components of the apparatus are corroded to damage the function. Further, a high ozone density environment is not good for operators.
In deep ultraviolet rays such as an excimer laser of having a wavelength of 250 nm or shorter than this, particularly, an ArF excimer laser having a wavelength of about 193 nm, there are plural oxygen (O2) absorbing zones in the bandwidth near the wavelength described above. Thus, in response to absorption of light by oxygen, ozone is generated. The above-described problem is, therefore, quite notable in the wavelength range of 250 nm or less.
It is an object of the present invention to provide an exposure apparatus of high durability by which the problem of deterioration of components by the production of ozone, which is quite notable in the wavelength region of 250 nm, is overcome.
It is another object of the present invention to provide a device manufacturing method which is based on such an exposure apparatus.